GENERAL STRUCTURE. IO47 



or two exceptions, is completely ossified ; and, in conjunction with 

 the exoccipitals, forms the single occipital condyle by which the 

 cranium articulates with the atlas vertebra. In the adult there is, 

 as a rule, no distinct parasphenoid 1 on the base of the skull. The 

 mandibular rami respectively consist of an articular cartilage-bone and 

 of several membrane-bones ; the articular bone being connected with 

 the squamosal of the cranium by a quadrate. The apparent ankle- 

 joint, in all existing forms, is situated between the proximal and 

 distal rows of the tarsus ; and not, as in the Mammalia, between 

 the tibia and astragalus. Gills are never developed during any 

 period of life ; the embryo is provided with an amnion and an 

 allantois ; and there are no mammary glands. As the palaeontologist 

 will not have to deal with the other distinctive features derived from 

 the soft parts, it will not be necessary to refer to them in this work. 



Regarding the features of Reptiles as distinct from Birds, the 

 approximation between the two classes is so close that it is difficult 

 to give any very clear diagnosis. In the present class, however, the 

 epidermal structures take the form of overlapping horny scales 

 (Squamata), or of shields with their edges in apposition (Chelonia) ; 

 while dermal bony scutes are very frequently developed. The ver- 

 tebrae may be amphiccelous, opisthocoelous, or proccelous ; but the 

 centra of the cervicals do not have cylindroidal and saddle-shaped 

 articular surfaces. The sacral vertebrae, when present, have broad 

 expanded ribs for articulation with the ilia. The sternum in exist- 

 ing forms is rhomboidal ; and the ribs may be attached to it by a 

 backward median process, or processes. The interclavicle is never 

 fused with the clavicles. There are more than three digits in the 

 manus ; and never less than three in the pes. Except in the Thero- 

 morous branch, the three elements of the pelvis as a rule 

 remain distinct ; 2 and there is apparently no known instance, except 

 one which may probably be regarded as a pathological peculiarity, 

 of the fusion of the metatarsals, or of their union with the distal row 

 of the tarsus. In all living Reptiles there is both a right and a left 

 aortic arch ; the arterial and venous circulations are at best but im- 

 perfectly separated ; and the blood is cold. 



Since the various orders of Reptiles differ so greatly from one 

 another in structure it will be advisable to make most of our obser- 

 vations on their osteology under those several headings. As a rule, 

 however, the bones of the cranium retain the general arrangement 

 observable in the Amphibia ; there being distinct postorbital or 

 postfrontal ossifications ; usually either one or two temporal arcades ; 

 and distinct post-, supra-, and infratemporal fossae, as defined in the 



1 Dr Baur suggests that this bone may be present in Palceohatteria. 



2 They are anchylosed in Testudo atlas of the Indian Siwaliks, and also in 

 Miolania. 



VOL. II. L 



